Method of forming an oxygen- or nitrogen-terminated silicon nanocrystalline structure and an oxygen- or nitrogen-terminated silicon nanocrystalline structure formed by the method

ABSTRACT

A substrate is set at a predetermined temperature in a plasma treatment chamber, then the inside of the plasma treatment chamber is regulated at a reduced pressure containing at least a silicon hydride gas and a hydrogen gas, a high-frequency electric field is applied to form a silicon film of nanometer scale thickness composed of fine silicon crystals and amorphous silicon on the substrate. Thereafter, application of the high-frequency electric field is terminated, then the inside of the plasma treatment chamber is replaced by an oxidizing or nitriding gas, and a high-frequency electric field is applied again for plasma oxidizing treatment or plasma nitriding treatment of the silicon film formed on the substrate. Thereby, a silicon nanocrystalline structure can be formed on a silicon substrate by using a process of producing silicon integrated circuits with achieving high luminous efficiency, and terminating reliably with oxygen or nitrogen on the surface thereof. According to the method of the present invention, the particle diameter of the oxygen- or nitrogen-terminated silicon nanocrystals can be regulated in an accuracy of 1 to 2 nm, the density thereof per unit area can be increased, and the silicon nanocrystalline structure can be produced easily and inexpensively.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of forming an oxygen- ornitrogen-terminated silicon nanocrystalline structure (porous silicon)and in particular to a novel method of forming a silicon nanocrystallinestructure (porous silicon) by using a plasma treatment equipment and toa silicon nanocrystalline structure (porous silicon) formed by themethod.

2. Description of the Related Art

Semiconductors of compounds such as gallium and arsenic would beapplicable as silicon-based light-emitting elements used in the fieldsof optical interconnection for optical communication, opticalcommunication and visible-light sources.

However, semiconductors of compounds such as gallium and arsenic hardlyproduce a structure with fewer defects on a silicon substrate, and arepoor in thermostability. Because the existing process of producingsilicon integrated circuits can not deal with such production, a newproduction process is necessary, thus increasing production costs.

Accordingly, there is demand for techniques of producing silicon-basedlight-emitting structures that can be produced by only the existingprocess of producing silicon integrated circuits.

As a silicon-based light-emitting material, a silicon nanocrystallinestructure (porous silicon) produced by anodizing is known.

A flow of anodizing treatment, i.e. a conventional process for producinga silicon nanocrystalline structure (porous silicon), is shown in FIG.5.

In FIG. 5, a polysilicon film is formed to a thickness of about 1 pm ona substrate by CVD (Chemical Vapor Deposition) method, and then thepolysilicon film formed by CVD method is subjected to anodizingtreatment.

For example, when an electrolytic cell filled with an electrolyte suchas an aqueous solution of HF, wherein a semiconductor such as siliconsubstrate is used as an anode and platinum is used as a cathode, iselectrified, electrons are transferred from the anode through anexternal circuit to the cathode, and oxidation reaction in a generalmeaning proceeds on the surface of the anode (i.e. the siliconesubstrate) contacting with the electrolyte.

However, a silicon nanocrystalline structure (porous silicon) producedby the above-mentioned conventional method has problems such as lowluminous efficiency and a low luminous extinction rate in the order ofmicrosecond (μsec). Further, the production process is complicatedbecause of its wet process involving an electrochemical treatment, andis hardly applied to a process of producing silicon integrated circuits.There is also a problem that the surface of silicon nanocrystallinestructure (porous silicon) thus produced is covered with hydrogen atoms,and it is unstable in composition, and fragile, so that it is easilybroken.

Japanese Patent Application Laid-Open (JP-A) No. 2000-273450 proposes asilicon nanocrystalline structure (porous silicon) that can be produceddirectly on a substrate such as a silicon substrate by the process ofproducing silicon integrated circuits. In this prior art, asilicon-based light-emitting material having a silicon-rich amorphousstructure, based on silicon and nitrogen, is produced from a rawmaterial gas such as silane (SiH₄) and an ammonia gas (NH₃) in aspecific ratio of raw material g/(raw material gas+ammonia gas) by CVDmethod at specific temperature.

The conventional method of forming a silicon nanocrystalline structure(porous silicon), shown in the treatment flow in FIG. 5, involvesformation, by chemical synthesis, of a nanocrystalline structure of apolysilicon film of 1 μm in thickness produced by CVD and simultaneousoxygen termination of the surface of the said polysilicon film. That is,the formation of a nanocrystalline structure by a polysiliconoxidation/etching process and the oxygen termination of the surface byan oxidization process proceed simultaneously in the chemical treatment.Due to this production process, the particle diameter of theoxygen-terminated silicon nanocrystalline structure is hardly regulatedin an accuracy of 1 to 2 nm, thus causing problems such as varyingluminescent colors.

The before described JP-A No. 2000-273450 does not refer to theregulation of the particle diameter of the silicon nanocrystallinestructure (porous silicon) affecting luminescent colors.

Further, because nanocrystals formed during the chemical treatment maydisappear in the subsequent etching/oxidization process, there is theproblem of a reduction in density of nanocrystals per unit area. Thisproblem is a major cause of a reduction in luminous efficiency in aporous silicon luminescent system using the silicon nanocrystallinestructure (porous silicon) formed by the conventional method shown inthe treatment flow in FIG. 5.

SUMMARY OF THE INVENTION

Accordingly, the object of the present invention is to provide a siliconnanocrystalline structure (porous silicon) which can be produced easilyand inexpensively by proposing a novel method of forming a siliconnanocrystalline structure (porous silicon). According to the said novelmethod proposed by the present invention, a silicon nanocrystallinestructure (porous silicon) can be formed on a substrate such as asilicon substrate by using a process of producing silicon integratedcircuits. And, according to the said novel method, it can achieve highluminous efficiency, and the surface of formed silicon nanocrystallinestructure is terminated reliably with oxygen or nitrogen to stabilizeits composition. Also, the particle diameter of the oxygen- ornitrogen-terminated silicon nanocrystalline structure (porous silicon)can be regulated in an accuracy of 1 to 2 nm, and the density thereofper unit area can be increased by the said novel method.

To accomplish the before described object, one aspect of the presentinvention provides a method of forming an oxygen- or nitrogen-terminatedsilicon nanocrystalline structure, which comprises a step of forming asilicon film of nanometer scale thickness composed of fine siliconcrystals and amorphous silicon on a substrate and a step of oxidizing ornitriding the formed silicon film with ions and radicals formed from anoxidizing gas or a nitriding gas.

Another aspect of the present invention provides a method of forming anoxygen- or nitrogen-terminated silicon nanocrystalline structure, wherein a step comprising of the first step of forming a silicon film ofnanometer scale thickness composed of fine silicon crystals andamorphous silicon on a substrate and the sequential second step ofoxidizing or nitriding the formed silicon film with ions and radicalsformed from an oxidizing gas or a nitriding gas is conducted pluraltimes.

According to the latter method, it is possible to form a multi-layerfilm containing an oxygen-terminated silicon nanocrystalline structure,a multi-layer film containing a nitrogen-terminated siliconnanocrystalline structure, or a structure wherein a multi-layer filmcontaining an oxygen-terminated silicon nanocrystalline structure islaminated with a multi-layer film containing a nitrogen-terminatedsilicon nanocrystalline structure.

In the before described methods of the present invention, the step offorming a silicon film of nanometer scale thickness composed of finesilicon crystals and amorphous silicon on a substrate can make use ofeither of the following two methods.

A first method is a method wherein a silicon film of nanometer scalethickness composed of fine silicon crystals and amorphous silicon isformed on a substrate by using a thermal catalysis reaction in a gassystem containing at least a silicon hydride gas and a hydrogen gas.

A second method is a method wherein a silicon film of nanometer scalethickness composed of fine silicon crystals and amorphous silicon isformed on a substrate by setting the substrate at a predeterminedtemperature in a plasma treatment chamber and then applying ahigh-frequency electric field while controlling the inside of the plasmatreatment chamber at a reduced pressure and containing at least asilicon hydride gas and a hydrogen gas.

A schematic diagram of the silicon film of nanometer scale thicknesscomposed of fine silicon crystals and amorphous silicon formed on asubstrate by these steps is shown in FIG. 1(a).

When the latter method (second method) is used, a VHF-range highfrequency having a higher frequency than 60 MHz can be adopted as thehigh frequency of the high-frequency electric field applied in the stepof forming a silicon film of nanometer scale thickness composed of finesilicon crystals and amorphous silicon on a substrate. The VHF-rangehigh frequency having a higher frequency than 60 MHz is advantageous forpromoting dissociation of hydrogen and oxygen.

When the former method (first method) is used, a thermal catalysisreaction chamber can be used as a reaction chamber for forming a siliconfilm of nanometer scale thickness composed of fine silicon crystals andamorphous silicon on a substrate. As the thermal catalyst reactionchamber, a chamber proposed in e.g. JP-A No. 2000-277501 can be used.That is, a thermal catalyst reaction chamber for a chemical vapordeposition (CVD) equipment can be used. This thermal catalyst reactionchamber comprises a treatment container in which a predeterminedtreatment such as formation of a film on a substrate is conducted, a gasfeeding system supplying a predetermined raw material gas to the saidtreatment container, a heater such as tungsten arranged in the saidtreatment container so as to allow the supplied raw material gas to passthrough the surface of it, an energy supplying mechanism supplyingenergy to the heater so as to maintain the heater at a predeterminedhigh temperature, and a substrate holder maintaining a substrate at thatposition in the treatment container at which a predetermined film isformed on the said substrate by decomposition or activation of the rawmaterial gas on the surface of the heater maintained at a predeterminedhigh temperature (for example, 1500 to 1900° C.). As the raw materialgas, a gas containing at least silicon hydride gas and a hydrogen gas isused.

It is well-known to engineers participating in a technology for growthof amorphous silicon or polysilicon that a silicon film of nanometerscale thickness composed of fine silicon crystals and amorphous siliconcan be formed on a substrate by using a thermal catalyst reaction in agas system containing at least a silicon hydride gas and a hydrogen gas.

In either of the above methods of forming an oxygen- ornitrogen-terminated silicon nanocrystalline structure, the step ofoxidizing or nitriding the silicon film formed on a substrate with ionsand radicals formed from an oxidizing gas or a nitriding gas can beconducted by plasma oxidizing treatment or plasma nitriding treatment ofthe silicon film by arranging the substrate having the silicon filmformed thereon in a plasma treatment chamber, replacing the atmospherein the plasma treatment chamber by an oxidizing or nitriding gasatmosphere and then applying a high-frequency electric field.

In this case, the plasma oxidizing treatment or plasma nitridingtreatment can be composed of plasma oxidizing treatment or plasmanitriding treatment in an oxidizing or nitriding gas atmosphere,subsequent etching treatment, with an HF-based gas, of the surface offine silicon crystals in the silicon film formed on the substrate, orplasma etching treatment of the surface of fine silicon crystals in thesilicon film formed on the substrate in a molecular gas systemcontaining fluorine, and subsequent plasma oxidizing treatment or plasmanitriding treatment in an oxidizing or nitriding gas atmosphere. Thismethod is advantageous because termination with oxygen or nitrogen canbe completely achieved although the treatment time is longer.

In the plasma oxidizing treatment or plasma nitriding treatment, theoxidizing or nitriding rate of amorphous silicon regions in the siliconfilm of nanometer scale thickness composed of fine silicon crystals andamorphous silicon is significantly higher than in crystalline regions.This is because many Si—H linkages occur in the amorphous siliconregions. Accordingly, the silicon crystalline regions can be terminatedthroughout with oxygen or nitrogen as shown in FIG. 1(b).

Examples of the usable oxidizing gas include an oxygen gas, and examplesof the usable nitriding gas include an ammonia gas and a nitrogen gas,etc.

Examples of the usable high-frequency, a high frequency having anLF-range high frequency applied to a VHF-range high frequency having ahigher frequency than 60 MHz can be used as the high frequency of thehigh-frequency electric field applied in the plasma oxidizing treatmentor plasma nitriding treatment.

Generally speaking, when plasma is formed by high-frequency electricpower, the acceleration of charged particles constituting the plasmacolliding against a substrate is decreased as the frequency of thehigh-frequency electric power is increased, and less damage is given tothe substrate. For promoting the dissociation of hydrogen and oxygen andsimultaneously increasing the acceleration of charged particlesconstituting the plasma colliding against a substrate, an LF-range highfrequency is superposed as second high frequency on the high frequency(for example the above-mentioned VHF-range high frequency having ahigher frequency than 60 MHz) from a high-frequency power source, toaccelerate the nitriding or oxidizing gas to collide against thesubstrate, whereby nitrogen or oxygen termination of the surface and theinside of the silicon film can be promoted.

Accordingly, a high frequency having an LF-range high frequency appliedto a VHF-range high frequency higher than 60 MHz is applied for theplasma oxidizing or plasma nitriding treatment, whereby the silicon filmof nanometer scale thickness composed of fine silicon crystals andamorphous silicon can be terminated more reliably with oxygen ornitrogen.

In the methods of forming an oxygen- or nitrogen-terminated siliconnanocrystalline structure as described above, the thickness of thesilicon film is desirably between 1 and 10 nm in the step of forming thesilicon film of nanometer scale thickness composed of fine siliconcrystals and amorphous silicon on a substrate. This is because thethickness of the silicon film in this range is preferable forefficiently proceeding the subsequent oxidizing treatment (for example,plasma oxidizing treatment) or nitriding treatment (for example, plasmanitriding treatment) of the silicon film with ions and radicals formedfrom an oxidizing gas or a nitriding gas.

Accordingly, the thickness of the silicon film is preferably thicker byabout 0.5 nm than the desired particle diameter of an oxygen- ornitrogen-terminated silicon nanocrystalline structure to be formed. Forexample, when an oxygen- or nitrogen-terminated silicon nanocrystallinestructure having a particle diameter of 3 nm is to be formed, thethickness of the silicon film is desirably 3.5 nm.

When a cluster-type unit system provided with two chambers, that is, achamber for forming a silicon film of nanometer scale thickness composedof fine silicon crystals and amorphous silicon on a substrate and achamber for plasma oxidizing treatment in an oxidizing gas atmosphere orfor plasma nitriding treatment in a nitriding gas atmosphere such asammonia gas is used. And if the step of forming a silicon film ofnanometer scale thickness composed of fine silicon crystals andamorphous silicon on a substrate is carried out by using a thermalcatalysis reaction in a gas system containing at least a silicon hydridegas and a hydrogen gas, constituted such that the above-describedthermal catalysis reaction chamber of the cluster-type unit system canbe used as a chamber for forming a silicon film of nanometer scalethickness composed of fine silicon crystals and amorphous silicon on asubstrate.

When the step of forming a silicon film of nanometer scale thicknesscomposed of fine silicon crystals and amorphous silicon on a substrateis carried out by setting the substrate at a predetermined temperaturein a plasma treatment chamber and then applying a high-frequencyelectric field while regulating the inside of the plasma treatmentchamber at a reduced pressure containing at least a silicon hydride gasand a hydrogen gas, the step of forming a silicon film of nanometerscale thickness composed of fine silicon crystals and amorphous siliconon a substrate by using the plasma treatment chamber and the subsequentstep of oxidizing or nitriding the formed silicon film with ions andradicals formed from an oxidizing gas or a nitriding gas can be carriedout by using the same plasma treatment chamber.

This can be carried out as follows. A substrate is set at apredetermined temperature in the plasma treatment chamber, then theinside of the plasma treatment chamber is regulated at a reducedpressure containing at least a silicon hydride gas and a hydrogen gas. Ahigh-frequency electric field is applied to form a silicon film ofnanometer scale thickness composed of fine silicon crystals andamorphous silicon on the substrate. After application of thehigh-frequency electric field is terminated, replacing the atmosphere inthe plasma treatment chamber by an oxidizing or nitriding gasatmosphere. And a high-frequency electric field is applied again forplasma oxidizing treatment or plasma nitriding treatment of the siliconfilm formed on the substrate.

The thermal catalyst reaction chamber is advantageously used as achamber for forming a silicon film of nanometer scale thickness composedof fine silicon crystals and amorphous silicon on a substrate becausethe thermal catalyst reaction chamber is more inexpensive than theplasma treatment chamber, to reduce production costs.

The silicon nanocrystalline structure proposed by the present invention,that is, the porous silicon, is formed by any one of the above methodsof forming an oxygen- or nitrogen-terminated silicon nanocrystallinestructure according to the present invention, and can be used widely asa silicon nanoluminous element.

That is, when a silicon nanocrystalline structure (porous silicon) isutilized as a luminous element, the visible emission range is a problem.But, in the present invention, a silicon film of nanometer scalethickness in the range of several nm to several tens nm composed of finesilicon crystals and amorphous silicon can be formed on a substrate toregulate the particle diameter of fine silicon crystals in the film. Sothat, the silicon nanocrystalline structure (porous silicon) applicableto luminous elements can be provided by the present invention.

According to the method of forming a silicon nanocrystalline structurein the present invention, a silicon nanocrystalline structure can beformed on a substrate such as a silicon substrate by using a process ofproducing silicon integrated circuits. Also, according to the method ofthe present invention, it can achieve high luminous efficiency, and thesurface of formed silicon nanocrystalline structure can be terminatedreliably with oxygen or nitrogen. And, according to the method of thepresent invention, the particle diameter of the oxygen- ornitrogen-terminated silicon nanocrystals can be regulated in an accuracyof 1 to 2 nm, and the density thereof per unit area can be increased.Further, the silicon nanocrystalline structure can be produced easilyand inexpensively.

Thus, the regulation of nanometer scale thickness in formation of anoxygen-terminated. silicon nanocrystalline structure is significantlyimproved as compared with that in the prior art, and thus theperformance of elements using the oxygen- or nitrogen-terminated siliconnanocrystalline structure formed by the method of the present inventioncan be significantly improved.

Further, the process from the step of forming a silicon film ofnanometer scale thickness composed of fine silicon crystals andamorphous silicon on a substrate to the step of oxygen or nitrogentermination treatment can be carried out in one unit system. As aresult, the unit cost can be reduced.

According to the method of forming a silicon nanocrystalline structurein the present invention, a nitrogen-terminated silicon nanocrystallinestructure can also be easily formed, and thus new application of thesilicon nanocrystalline structure can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) and 1(b) show a diagram of a process of forming a siliconnanocrystalline structure by the method of the present invention, andFIG. 1(a) is a diagram showing a process of forming a silicon film ofnanometer scale thickness composed of fine silicon crystals andamorphous silicon on a substrate, and FIG. 1(b) is a drawing showing thestate of an oxygen- or nitrogen-terminated silicon nanocrystallinestructure formed by selectively oxidizing or nitriding amorphous siliconregions;

FIG. 2 is a longitudinal section of a partially omitted internalstructure of a plasma treatment equipment used in the method of thepresent invention;

FIG. 3 is a longitudinal section of a partially omitted internalstructure in another state of the plasma treatment equipment shown inFIG. 2;

FIG. 4 is a schematic top plane view of a system structure having athermal catalysis treatment chamber and a plasma treatment chamberintegrated as a cluster-type unit system for carrying out the method ofthe present invention; and

FIG. 5 shows a flow of a conventional process of forming anoxygen-terminated silicon nanocrystalline structure (porous silicon).

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention and preferableexamples are described by reference to the accompanying drawings.

The plasma technology for regulating the nanometer scale thickness asdescribed in the present invention depends significantly on thestructure of the treatment equipment. Accordingly, one example of theplasma treatment equipment that can be used in the method of forming anoxygen- or nitrogen-terminated silicon nanocrystalline structureaccording to the present invention is described in more detail.

FIGS. 2 and 3 show respectively a sectional view showing an internalstructure of a plasma treatment equipment that can be used in the methodof forming an oxygen- or nitrogen-terminated silicon nanocrystallinestructure according to the present invention.

A plasma treatment unit 31 has a parallel plate-type electrode structurecomposed of an upper electrode 13 and a lower electrode 14.

The upper electrode 13 is an electrode to which two types of superposedhigh frequencies are fed via a matching circuit 34 from a high-frequencypower source 32 for VHF-range high frequency and an LF-rangehigh-frequency power source 33 for LF-range high frequency.

The lower electrode 14 is an earth electrode forming a substrate holderstage serving as a substrate-loading part.

The lower electrode 14 is provided with a vertically moving mechanism35, and by the vertically moving mechanism 35, the lower electrode 14 isvertically transferred. In FIG. 2, the lower electrode 14 is in thehighest position, and in FIG. 3, the lower electrode 14 is in the lowestposition.

In the illustrated example, a reaction chamber 11 for plasma treatmentis made of a closed structure, and the inside thereof is set in apredetermined vacuum (reduced pressure).

The reaction chamber 11 is made of a metallic material, and haselectrical conductivity. The reaction chamber 11 is provided with a portfor introducing or discharging a substrate 12 as the object oftreatment, an exhaust port and evacuation system for achieving apredetermined vacuum therein, a gas-introducing mechanism forintroducing a discharging gas causing discharge, etc., but theseelements are those of known structures for the plasma treatmentequipment and not important for describing the method of the presentinvention, and for the convenience of the description, these elementsare not shown in FIGS. 2 and 3.

The reaction chamber 11 is composed of a cylindrical side member 41, aceiling member 42 and a bottom member 43. The reaction chamber 11 isgrounded and maintained at ground potential. The bottom member 43 issupported by a plurality of supporting columns 44 supporting the wholeof the reaction chamber 11. The center of the ceiling member 42 isformed with an opening, and this opening is provided via a ring-shapedinsulator 45 with the upper electrode 13 by a bolt 46. The upperelectrode 13 is composed of an upper member 13 a and a lower member 13b. A high frequency transmission cable 17 is connected to a connectingterminal 47 arranged on the center of the top of the upper member 13 a.The lower member 13 b is fixed by a screw 48 to the bottom of the uppermember 13 a. By the screw 48, the ring-shaped insulator 49 is alsoattached to the periphery of the bottom of the lower member 13 b in theupper electrode 13. A gas passage 50 is formed in a space between theupper member 13 a and the lower member 13 b, and inside of the uppermember 13 a. A process gas can flow through the said gas passage 50. Thegas introducing mechanism for introducing a discharging gas into the gaspassage 50 is not shown in the drawing.

The upper electrode 13 and the lower electrode 14 are basically in theform of an electrically conductive plate that is circular as a whole,and are arranged parallel and facing with each another with a desireddistance. The distance between the upper electrode 13 and the lowerelectrode 14 can be arbitrarily changed by changing the position of thelower electrode 14 by the vertically moving mechanism 35.

The upper electrode 13 is connected via the matching circuit 34 with thehigh-frequency power source 32 and the LF-range high-frequency powersource 33. The high-frequency power source 32 is a power source forgenerating high frequency in the VHF range, while the LF-rangehigh-frequency power source 33 is a power source for generating LF highfrequency in the LF range. The high frequency generated from thehigh-frequency power source 32 is preferably 60 MHz, while the highfrequency from the LF-range high-frequency power source 33 is preferably400 KHz. The high frequencies generated from the power sources 32 and 33are superposed in the matching circuit 34, and then supplied in thisstate to the upper electrode 13. The high frequency and LF-range highfrequency from the power sources 32 and 33 are supplied to the upperelectrode 13 via the cable 17 and the connecting terminal 47. The highfrequency supplied to the upper electrode 13 becomes the energy of majordischarge generated in the gap between the upper electrode 13 and thelower electrode 14.

The high-frequency power source 32 supplies electric power for excitingplasma discharge. The LF-range high-frequency power source 33 givesself-bias voltage determining the collision energy of positive ions.

When the lower electrode 14 is in the highest position as shown in FIG.2, the lower electrode 14 is elevated and transferred upwards, and thusthe substrate 12 is loaded on the top of the lower electrode 14 incontact therewith.

A ring-shaped first insulator 52, a donut disk-shaped second insulator53, a ring-shaped third insulator 54 and a cylindrical fourth insulator55 are arranged on the backside of the lower electrode 14. The back ofthe lower electrode 14 and the whole surface of support 19 are coveredwith the first to fourth insulators 52 to 55. Further, the surfaces ofthe second to fourth insulators 53 to 55 are covered with twoelectroconductive members 56 and 57. The whole surfaces of theinsulators 52 to 55 exposed in the reaction chamber 11, excluding theperipheries of the first and second insulators 52 and 53, are coveredwith the electroconductive members 56 and 57. The support 19 for thelower electrode 14 is rod-shaped and composed of an electroconductivemember. The lower end of the support 19 is provided with anelectroconductive flange 61. The support 19 for the lower electrode 14and its related portions are arranged so as to extend via an opening 43a formed in the center of the bottom member 43 to the bottom of thereaction chamber 11. These portions are enclosed with a bellows 21attached to the bottom of the bottom member 43 so as to cover theoutside of the opening 43 a formed in the bottom member 43.

In the method of forming an oxygen- or nitrogen-terminated siliconnanocrystalline structure according to the present invention, and incase the step of forming a silicon film of nanometer scale thicknesscomposed of fine silicon crystals and amorphous silicon on a substrateis carried out by setting the substrate at a predetermined temperaturein a plasma treatment chamber and then applying a high-frequencyelectric field while regulating the inside of the plasma treatmentchamber at a reduced pressure containing at least a silicon hydride gasand a hydrogen gas, plasma excited by high frequency, for example plasmaexcited by VHF-range high frequency higher than 60 MHz, can be stablyformed by using the structure of the plasma treatment equipmentdescribed above.

EXAMPLES

Then, preferable examples of the present invention using the plasmatreatment equipment described above are described. In these examples, an8-inch silicon substrate is used. However, the plasma treatmentequipment may be designed so as to treat a 1 m square substrate, and themethod of the present invention can be applied to the such largesubstrate.

Example 1

As a first example, an example of formation of an oxygen-terminatedsilicon nanocrystalline structure is described.

First, the silicon substrate 12 was placed on the lower electrode 14 inthe position shown in FIG. 3, and then the lower electrode 14 waselevated to the position shown in FIG. 2 until the distance between theupper electrode 13 and the lower electrode 14 was reduced for example to3 cm.

Thereafter, the silicon substrate 12 was set at a predeterminedtemperature. For example, the temperature of the silicon substrate 12was increased to the same temperature (for example, 350° C.) as that ofthe lower electrode 14 and stabilized at that temperature. In this case,stabilization was attained in a short time by introducing a hydrogengas.

Subsequently, a monosilane (SiH₄) gas, 0.026 mg/sec. (2 sccm), was mixedwith a hydrogen gas, 0.95 mg/sec. (500 sccm) and introduced as a siliconhydride gas into the reaction chamber 11. And then, applying 600 MHzhigh-frequency power of 500 W with regulating the pressure at 10 Pa toform a silicon film of nanometer scale thickness composed of finesilicon crystals and amorphous silicon on the substrate.

The thickness of the silicon film can be regulated by selecting thedeposition time, and in this example, the film was deposited for 5seconds to form a silicon film of 3.5 nm in thickness.

Subsequently, the gas in the reaction chamber 11 was discharged, and amixed gas of an oxygen gas (8.0 mg/sec. (500 sccm)) and an argon gas(3.4 mg/sec. (200 sccm)) was introduced as an oxidizing gas into thechamber 11. And then, superposing and applying 60 MHz high-frequencypower of 500 W and 400 KHz high-frequency power of 200 W with regulatingthe pressure at 10 Pa were superposed for plasma oxidizing treatment.

The role of the argon gas is that it is ionized in plasma to selectivelyremove, by its ion impact, Si—H bonds constituting amorphous siliconregions in a silicon film of nanometer scale thickness composed of finesilicon crystals and amorphous silicon.

Under the conditions in this example, an oxygen-terminated siliconnanocrystalline structure could be formed by the plasma oxidizingtreatment for 5 seconds.

Then, the before described silicon film-forming step and plasmaoxidizing step were repeated 300 times for about 50 minutes in total.That is, the first silicon film-forming step followed by the firstplasma oxidizing step was conducted, and then, the secondsilicon-forming step followed by the second plasma oxidizing step wasconducted, so as to repeat the before described silicon film-formingstep and plasma oxidizing step 300 times for about 50 minutes.Oxygen-terminated silicon nanocrystalline structure (porous silicon)about 1 μm was formed on the silicon substrate 12.

Example 2

Now, an example of formation of a nitrogen-terminated siliconnanocrystalline structure is described.

Similar to the first example, the silicon substrate 12 was placed on thelower electrode 14 in the position shown in FIG. 3, and then the lowerelectrode 14 was elevated to the position shown in FIG. 2 until thedistance between the upper electrode 13 and the lower electrode 14 wasreduced for example to 3 cm.

Thereafter, the silicon substrate 12 was set at a predeterminedtemperature. For example, the temperature of the silicon substrate 12was increased to the same temperature (for example, 350° C.) as that ofthe lower electrode 14 and stabilized at that temperature. In this case,stabilization was attained in a short time by introducing a hydrogengas.

Subsequently, a monosilane (SiH₄) gas, 0.026 mg/sec. (2 sccm), was mixedwith a hydrogen gas, 0.95 mg/sec. (500 sccm) and introduced as a siliconhydride gas into the reaction chamber 11. And then, applying 60 MHzhigh-frequency power of 500 W with regulating the pressure at 10 Pa toform a silicon film of nanometer scale thickness composed of finesilicon crystals and amorphous silicon on the substrate. In the secondexample too, the film was deposited for 5 seconds to form a silicon filmof 3.5 nm in thickness.

Subsequently, the gas in the reaction chamber 11 was discharged, and amixed gas of an ammonia gas (4.9 mg/sec. (500 sccm)) and an argon gas (3.4 mg/sec. (200 sccm)) was introduced as a nitriding gas into thechamber 11 And then, superposing and applying 60 MHz high-frequencypower of 700 W and 400 KHz high-frequency power of 200 W with regulatingthe pressure at 10 Pa for plasma nitriding treatment.

The reason for use of higher 60 MHz high-frequency power in this examplethan in the plasma oxidizing treatment in the first example is that thereaction energy is higher in the nitriding process.

In the nitriding treatment in the second example, a nitrogen-terminatedsilicon nanocrystalline structure could be formed by the plasmatreatment for 5 seconds.

Then, as similar to the first example, the before described siliconfilm-forming step and plasma nitriding step were repeated 300 times forabout 5.0 minutes in total. Nitrogen-terminated silicon nanocrystallinestructure (porous silicon) about 1 μm was formed on the siliconsubstrate 12.

Example 3

In this example, after the step of forming a silicon film of nanometerscale thickness composed of fine silicon crystals and amorphous siliconon a substrate, the plasma oxidizing treatment or plasma nitridingtreatment of the silicon film formed on the substrate was carried out byplasma oxidizing treatment or plasma nitriding treatment in an oxidizingor nitriding gas atmosphere. And then, etching treatment, with anHF-based gas, of the surface of fine silicon crystals in the siliconfilm formed on the substrate, or plasma etching treatment of the surfaceof fine silicon crystals in the silicon film formed on the substrate ina molecular gas system containing fluorine was carried out. And,subsequently, plasma oxidizing treatment or plasma nitriding treatmentin an oxidizing or nitriding gas atmosphere was carried out. The processof this example is described below.

Similar to the first example, the silicon substrate 12 was placed on thelower electrode 14 in the position shown in FIG. 3, and then the lowerelectrode 14 was elevated to the position shown in FIG. 2 until thedistance between the upper electrode 13 and the lower electrode 14 wasreduced for example to 3 cm.

Thereafter, the silicon substrate 12 was set at a predeterminedtemperature. For example, the temperature of the silicon substrate 12was increased to the same temperature (for example, 350° C.) as that ofthe lower electrode 14 and stabilized at that temperature. In this case,stabilization was attained in a short time by introducing a hydrogengas.

Subsequently, a monosilane (SiH₄) gas, 0.026 mg/sec. (2 sccm), was mixedwith a hydrogen gas, 0.95 mg/sec. (500 sccm) and introduced as a siliconhydride gas into the reaction chamber 11. And then, applying 600 MHzhigh-frequency power of 500 W with regulating the pressure at 10 Pa toform a silicon film of nanometer scale thickness composed of finesilicon crystals and amorphous silicon on the substrate. In the thirdexample too, the film was deposited for 5 seconds to form a silicon filmof 3.5 nm in thickness.

After the silicon film was formed, the gas in the reaction chamber 11was discharged, and the plasma oxidizing treatment described in thefirst example was conducted.

Subsequently, the gas in the reaction chamber 11 was discharged, and amixed gas of a carbon tetrafluoride (CF₄) gas (0.20 mg/sec. (5 sccm))and an argon gas (1.7 mg/sec. (100 sccm)) was introduced as an etchinggas into the chamber 11. And then, applying 60 MHz high-frequency powerof 300 W with regulating the pressure at 50 Pa to conduct etchingtreatment for 3 seconds thereby removing an oxide layer on the surface.

Subsequently, the gas in the reaction chamber 11 was discharged, and theplasma oxidizing treatment described in the first example was conductedagain.

Then, as similar to the first example, the before described siliconfilm-forming step and plasma oxidizing step were repeated 300 times forabout 70 minutes in total. Oxygen-terminated silicon nanocrystallinestructure about 1 μm was formed on the silicon substrate 12.

After formation of the silicon film and after the etching treatment, anitrogen-terminated silicon nanocrystalline structure can be formed onthe silicon substrate 12 by conducting the plasma nitriding treatmentdescribed in the second example in place of the plasma oxidizingtreatment described in the first example. In this case, it is not anoxide layer but a nitride layer that is removed by the etchingtreatment.

Example 4

Two chambers, that is, a chamber for forming a silicon film of nanometerscale thickness composed of fine silicon crystals and amorphous siliconon a substrate and a chamber for plasma oxidizing treatment in anoxidizing gas atmosphere or for plasma nitriding treatment in anitriding gas atmosphere were used to improve throughput capacity. Anexample of this process is described below.

In this case, the chamber for forming a silicon film of nanometer scalethickness composed of fine silicon crystals and amorphous silicon on asubstrate is preferably a chamber for thermal catalysis reaction. Thisis because the thermal catalysis treatment chamber is more inexpensivethan the plasma treatment chamber, thus reducing the cost of the unit.

FIG. 4 shows a system structure having a thermal catalysis treatmentchamber and a plasma treatment chamber integrated as a cluster-type unitsystem for this example.

As the plasma treatment chamber, the chamber shown in FIGS. 2 and 3 andused in Examples 1, 2 and 3 was used.

As the thermal catalyst reaction chamber, a thermal catalyst reactionchamber for a chemical vapor deposition equipment (CVD equipment) can beused. This thermal catalyst reaction chamber comprises a treatmentcontainer for predetermined treatment such as formation of a film on asubstrate is conducted, a gas feeding system supplying a predeterminedraw material gas to the said treatment container, a heater such astungsten arranged in the said treatment container so as to allow thesupplied raw material gas to pass through the surface of it, an energysupplying mechanism supplying energy to the heater so as to maintain theheater at a predetermined high temperature, and a substrate holdermaintaining a substrate at that position in the treatment container atwhich a predetermined film is formed by decomposition or activation ofthe raw material gas on the surface of the heater maintained at apredetermined high temperature (for example, 1500 to 1900° C.). Asproposed in JP-A No. 2000-277501 etc., such a chemical vapor depositionequipment (CVD equipment) is known in the art, and it is well-known toengineers participating in a technology for forming of amorphous siliconor polysilicon that a silicon film of nanometer scale thickness composedof fine silicon crystals and amorphous silicon can be formed on asubstrate by a thermal catalyst reaction in a gas system containing atleast a silicon hydride gas and a hydrogen gas, and thus a specificdescription thereof is omitted.

The following example shows the step of forming a silicon film ofnanometer scale thickness composed of fine silicon crystals andamorphous silicon on a substrate and the step of plasma oxidizingtreatment, and as a matter of course, it is possible to conduct nitrogentermination treatment or oxidation termination treatment and nitrogentermination treatment both involving etching treatment.

In FIG. 4, the silicon substrate 12 is placed in a delivery chamber 100.

First, the silicon substrate 12 was arranged in a substrate introductionchamber 130. A gate valve 131 for the substrate introduction chamber wasopened, and by a robot 101, the silicon substrate 12 was transferred tothe delivery chamber 100. The gate valve 131 for the substrateintroduction chamber was closed, then a gate valve 111 for the thermalcatalysis chamber was opened, the silicon substrate 12 was introducedinto the thermal catalysis treatment chamber 110, and the gate valve 111for the thermal catalysis chamber was closed.

When the temperature of the silicon substrate 12 in the thermalcatalysis treatment chamber 110 reached a predetermined value, forexample 300° C., a hydrogen gas (0.38 mg/sec. (200 sccm)) was sprayedonto the surface of a heater (tungsten) heated at 1800° C., to cause thecatalysis reaction of hydrogen. The resulting hydrogen radicals werereacted with a monosilane (SiH₄) gas (0.026 mg/sec. (2 sccm) )introduced into the surface of the substrate, to form a silicon film ofnanometer scale thickness which similar to the film in the firstexample, consisted of fine silicon crystals and amorphous silicon on thesilicon substrate 12.

Subsequently, the gas in the thermal catalysis treatment chamber 110 wasdischarged, then the gate valve 111 for the thermal catalysis treatmentchamber was opened, and the silicon substrate 12 was transferred to thedelivery chamber 100. Subsequently, the gate valve 121 for the plasmatreatment chamber was opened, the silicon substrate 12 was arranged inthe plasma treatment chamber 120, and the gate valve 121 for the plasmatreatment chamber was closed. After the gas in the plasma treatmentchamber 120 was discharged, a mixed gas of an oxygen gas (8.0 mg/sec.(500 sccm)) and an argon gas (3.4 mg/sec. (200 sccm)) was introduced asan oxidizing gas into the plasma treatment chamber 120. And then,superposing and applying 60 MHz high-frequency power of 500 w and 400KHz high-frequency power of 200 W the pressure 10 Pa for plasmaoxidizing treatment.

Then, as similar to the first example, the before described siliconfilm-forming step and the plasma oxidizing step were repeated about 300times. Oxygen-terminated silicon nanocrystalline structure about 1 μmwas formed on the silicon substrate 12.

Finally, the silicon substrate 12 was transferred to a substratedelivery chamber 140, to complete the treatment in the cluster unitsystem.

Example of Evaluation Test

To confirm the effect of the present invention, light-emitting elementsusing the silicon nanocrystalline structure (porous silicon) formed bythe method of the present invention were compared in luminous efficiencywith a silicon nanocrystalline structure in the prior art (anodizingtreatment).

The light-emitting element for evaluation was prepared by forming ananti-reflection transparent electroconductive film such as ITO film asan upper electrode on a silicon substrate provided thereon with theoxygen-terminated silicon nanocrystalline structure in Example 1, asilicon substrate provided thereon with the nitrogen-terminated siliconnanocrystalline structure in Example 2, and a silicon substrate providedthereon with the oxygen-terminated silicon nanocrystalline structure inExample 3, each respectively, and then forming an ohmic electrode of anelectroconductive material such as Au or Al on the backside of thesilicon substrate.

As a comparative example, a light-emitting element was prepared byforming an anti-reflection transparent electroconductive film such asITO film as an upper electrode on a silicon substrate provided thereonwith a silicon nanocrystalline structure according to the conventionalanodizing treatment, and then forming an ohmic electrode of anelectroconductive material such as Au or Al on the backside of thesilicon substrate.

Assuming that the luminous efficiency of the light-emitting elementusing a silicon nanocrystalline structure according to the conventionalanodizing treatment was 100%, a luminous efficiency of about 300% wasachieved by the light-emitting element using the silicon nanocrystallinestructure in Example 1 and 2, and a luminous efficiency of about 400% bythe light-emitting element using the silicon nanocrystalline structurein Example 3.

In the before described embodiments and the accompanying drawings of thepresent invention, the conditions described in the examples, etc., arethose summarized to such a degree that the present invention can beunderstood. Accordingly, the present invention is not limited to thebefore described embodiments and the examples, and can be carried outunder various conditions within the scope of the technical idea setforth in the claims.

1. A method of forming an oxygen- or nitrogen-terminated siliconnanocrystalline structure, which comprises a step of forming a siliconfilm of nanometer scale thickness composed of fine silicon crystals andamorphous silicon on a substrate and a step of oxidizing or nitridingthe formed silicon film with ions and radicals formed from an oxidizinggas or a nitriding gas.
 2. A method of forming an oxygen- ornitrogen-terminated silicon nanocrystalline structure, wherein a stepcomprising of the first step of forming a silicon film of nanometerscale thickness composed of fine silicon crystals and amorphous siliconon a substrate and the sequential second step of oxidizing or nitridingthe formed silicon film with ions and radicals formed from an oxidizinggas or a nitriding gas is conducted plural times.
 3. A method of formingan oxygen- or nitrogen-terminated silicon nanocrystalline structureaccording to claim 1, wherein the step of forming a silicon film ofnanometer scale thickness composed of fine silicon crystals andamorphous silicon on a substrate is carried out by using a thermalcatalysis reaction in a gas system containing at least a silicon hydridegas and a hydrogen gas.
 4. A method of forming an oxygen- ornitrogen-terminated silicon nanocrystalline structure according to claim2, wherein the step of forming a silicon film of nanometer scalethickness composed of fine silicon crystals and amorphous silicon on asubstrate is carried out by using a thermal catalysis reaction in a gassystem containing at least a silicon hydride gas and a hydrogen gas. 5.A method of forming an oxygen- or nitrogen-terminated siliconnanocrystalline structure according to claim 1, wherein the step offorming a silicon film of nanometer scale thickness composed of finesilicon crystals and amorphous silicon on a substrate is carried out bysetting the substrate at a predetermined temperature in a plasmatreatment chamber and then applying a high-frequency electric fieldwhile regulating the inside of the plasma treatment chamber at a reducedpressure containing at least a silicon hydride gas and a hydrogen gas.6. A method of forming an oxygen- or nitrogen-terminated siliconnanocrystalline structure according to claim 2, wherein the step offorming a silicon film of nanometer scale thickness composed of finesilicon crystals and amorphous silicon on a substrate is carried out bysetting the substrate at a predetermined temperature in a plasmatreatment chamber and then applying a high-frequency electric fieldwhile regulating the inside of the plasma treatment chamber at a reducedpressure containing at least a silicon hydride gas and a hydrogen gas.7. A method of forming an oxygen- or nitrogen-terminated siliconnanocrystalline structure according to claim 5, wherein the frequency ofthe high-frequency electric field applied in the step of forming asilicon film of nanometer scale thickness composed of fine siliconcrystals and amorphous silicon on a substrate is a VHF-range highfrequency having a higher frequency than 60 MHz.
 8. A method of formingan oxygen- or nitrogen-terminated silicon nanocrystalline structureaccording to claim 6, wherein the frequency of the high-frequencyelectric field applied in the step of forming a silicon film ofnanometer scale thickness composed of fine silicon crystals andamorphous silicon on a substrate is a VHF-range high frequency having ahigher frequency than 60 MHz.
 9. A method of forming an oxygen- ornitrogen-terminated silicon nanocrystalline structure according to claim1, wherein the step of oxidizing or nitriding the silicon film formed ona substrate with ions and radicals formed from an oxidizing gas or anitriding gas is plasma oxidizing treatment or plasma nitridingtreatment of the silicon film by arranging the substrate having thesilicon film formed thereon in a plasma treatment chamber, replacing theatmosphere in the plasma treatment chamber by an oxidizing or nitridinggas atmosphere and then applying a high-frequency electric field.
 10. Amethod of forming an oxygen- or nitrogen-terminated siliconnanocrystalline structure according to claim 2, wherein the step ofoxidizing or nitriding the silicon film formed on a substrate with ionsand radicals formed from an oxidizing gas or a nitriding gas is plasmaoxidizing treatment or plasma nitriding treatment of the silicon film byarranging the substrate having the silicon film formed thereon in aplasma treatment chamber, replacing the atmosphere in the plasmatreatment chamber by an oxidizing or nitriding gas atmosphere and thenapplying a high-frequency electric field.
 11. A method of forming anoxygen- or nitrogen-terminated silicon nanocrystalline structureaccording to claim 9, wherein the plasma oxidizing treatment or plasmanitriding treatment is composed of plasma oxidizing treatment or plasmanitriding treatment in an oxidizing or nitriding gas atmosphere,subsequent etching treatment, with an HF-based gas, of the surface offine silicon crystals in the silicon film formed on the substrate, orplasma etching treatment of the surface of fine silicon crystals in thesilicon film formed on the substrate in a molecular gas systemcontaining fluorine, and subsequent plasma oxidizing treatment or plasmanitriding treatment in an oxidizing or nitriding gas atmosphere.
 12. Amethod of forming an oxygen- or nitrogen-terminated siliconnanocrystalline structure according to claim 10, wherein the plasmaoxidizing treatment or plasma nitriding treatment is composed of plasmaoxidizing treatment or plasma nitriding treatment in an oxidizing ornitriding gas atmosphere, subsequent etching treatment, with an HF-basedgas, of the surface of fine silicon crystals in the silicon film formedon the substrate, or plasma etching treatment of the surface of finesilicon crystals in the silicon film formed on the substrate in amolecular gas system containing fluorine, and subsequent plasmaoxidizing treatment or plasma nitriding treatment in an oxidizing ornitriding gas atmosphere.
 13. A method of forming an oxygen- ornitrogen-terminated silicon nanocrystalline structure according to claim9, wherein the high frequency of the high-frequency electric fieldapplied in the plasma oxidizing treatment or plasma nitriding treatmentis a high frequency having an LF-range high frequency applied to aVHF-range high frequency having a higher frequency than 60 MHz.
 14. Amethod of forming an oxygen- or nitrogen-terminated siliconnanocrystalline structure according to claim 10, wherein the highfrequency of the high-frequency electric field applied in the plasmaoxidizing treatment or plasma nitriding treatment is a high frequencyhaving an LF-range high frequency applied to a VHF-range high frequencyhaving a higher frequency than 60 MHz.
 15. A method of forming anoxygen- or nitrogen-terminated silicon nanocrystalline structureaccording to claim 11, wherein the high frequency of the high-frequencyelectric field applied in the plasma oxidizing treatment or plasmanitriding treatment is a high frequency having an LF-range highfrequency applied to a VHF-range high frequency having a higherfrequency than 60 MHz.
 16. A method of forming an oxygen- ornitrogen-terminated silicon nanocrystalline structure according to claim12, wherein the high frequency of the high-frequency electric fieldapplied in the plasma oxidizing treatment or plasma nitriding treatmentis a high frequency having an LF-range high frequency applied to aVHF-range high frequency having a higher frequency than 60 MHz.
 17. Amethod of forming an oxygen- or nitrogen-terminated siliconnanocrystalline structure according to claim 1, wherein in the step offorming a silicon film of nanometer scale thickness composed of finesilicon crystals and amorphous silicon on a substrate, the thickness ofthe silicon film is from 1 to 10 nm.
 18. A method of forming an oxygen-or nitrogen-terminated silicon nanocrystalline structure according toclaim 2, wherein in the step of forming a silicon film of nanometerscale thickness composed of fine silicon crystals and amorphous siliconon a substrate, the thickness of the silicon film is from 1 to 10 nm.19. A silicon nanocrystalline structure formed by the method of formingan oxygen- or nitrogen-terminated silicon nanocrystalline structure asdescribed in claim
 1. 20. A silicon nanocrystalline structure formed bythe method of forming an oxygen- or nitrogen-terminated siliconnanocrystalline structure as described in claim 2.